Optics is the science of light transmission through media. One of the most important examples of optics is the transmission of light through a waveguide which may be made from different materials such as glass, plastic, semiconductor material, or other light transparent material. Some of these materials may be made into fibers called optical fibers. An optical fiber is one type of waveguide.
Modulated light, such as a short optical pulse, is made up of different wavelengths of light. These different wavelengths may be affected differently by the medium through which the light passes. Dispersion is the process by which radiation (e.g., light) is separated in accordance with some characteristic (e.g., wavelength, polarization, etc.) into components which have different propagating characteristics. For example, an optical signal may be composed of three different wavelengths of light (e.g., A, B, and C). At any point in time, the order of these waveforms may be ABC. The medium through which the optical signal travels may cause the three waveforms to travel at different speeds and, therefore, cause the ordering of its components to be different when seen by a receiver. For example, a signal with the original waveform order ABC may be changed by the medium through which the signal travels to a signal of waveform order BAC by the time the signal reaches the receiver. For example, dispersion may cause a short optical pulse, composed of simultaneously launched wavelength components, to spread into a longer pulse because the components travel at different speeds and, therefore, arrive at the destination at different times. Such pulse spreading can be very harmful to the operation of optical systems (e.g., optical communications, high-speed switching, etc.). To reverse this process, dispersion compensation may be accomplished by properly delaying each component of the optical signal in order to restore the desired temporal alignment.
An optical fiber is a transparent, dielectric cylinder surrounded by a second transparent dielectric cylinder. The fiber is one type of waveguide used to transmit energy at optical wavelengths. The light travels along the length of the fiber and is kept within the fiber by a series of grazing-angle reflections from wall to wall of an interface between the inner dielectric cylinder (i.e., the core) and the outer dielectric cylinder (i.e., the cladding). The reflections are made possible by a high refractive index of the core and a lower refractive index of the cladding. As the light travels along the fiber, the differences in the refractive indices cause the light wave to bounce from the core/cladding interface on one side of the optical fiber back through the core to the opposite core/cladding interface. The core/cladding interface acts as a mirror to reflect without power loss and keep the light in the channel determined by the core.
There are two classes of optical fiber (i.e., single mode fiber and multi-mode fiber). A single mode fiber has a much smaller core than does a multi-mode fiber. With a wider core, a multi-mode fiber may transmits signals having a wider angle of incidence with respect to the axis of the fiber than does a single-mode fiber. Signals of different angles of incidence travel along the fiber with different angles of reflection and are said to be of different modes. Having a smaller core, a single-mode fiber only transmits optical signals that have an angle of incidence with respect to the fiber in a narrow range (effectively parallel with the core). Therefore, a single-mode fiber is thought of as accepting optical signals having a single angle of incidence with respect to the fiber (i.e., a single mode) while a multi-mode fiber transmits optical signals having many different angles of incidence with respect to the fiber (i.e., multiple modes). The speed with which an optical signal travels along an optical fiber depends on the signal's angle of incidence with respect to the fiber. For example, an optical signal that has an angle of incidence with respect to the fiber of zero degrees (i.e., parallel to the core) will take the least time to travel a distance along the fiber because the path through the fiber is the shortest (i.e., straight through). An optical signal having the greatest allowable angle of incidence with respect to the fiber will take the longest time to travel along the same length of fiber because the signal bounces back and forth from cladding interface to cladding interface, as it travels. Such a path is the longest path possible through the fiber and, therefore, takes the longest time to travel the length of the fiber.
The various modes of propagation that a light wave may take through an optical fiber may be designated by "LP.sub.1m," where "LP" stands for linearly polarized, where "1" is one-half the number of minima (or maxima) that occur around the circumference of the core in an intensity pattern of the particular light wave in question, and where "m" is the number of maxima in the intensity pattern that occur along a radial line between the core center and the outer surface of the fiber. A light wave may be transmitted in any one of a number of modes, where each mode is linearly polarized in one direction.
Various devices may be used to modulate a signal. Some of these devices include the term "chirp." A chirped grating is one where the spacing between each grating structure varies from structure to structure in some smooth fashion. Dispersion may be added to an optical signal by applying the signal to a chirped grating.
A prior art method of imposing time delay is to switch light into selectable fiber, or non-fiber waveguide, delay loops. The disadvantage of this approach is that the selection of delay is, necessarily, discrete (i.e., light is sent to a selected loop or not), and the method is not useful for imposing a gradiently changing delay (e.g., for dispersion compensation in an input with a broad, continuous spectrum). Dispersion compensation has been done by passing the signal through a medium with an opposing dispersion. For example, a length of dispersion compensating fiber (DCF) is spliced into a transmission fiber so that the optical dispersion of the DCF is of opposite sign to the optical dispersion of the transmission fiber. The disadvantage of this approach is that the obtainable delay functions are limited to those possible with dispersion compensating fiber, or non-fiber waveguide, and are either monotonically increasing or decreasing rather than user-definable. These ideas are disclosed in the following articles and patents: 1) "A compact device for highly efficient dispersion in fiber transmission," by Peschel et al., published in "Appl. Phys. Lett." 67 (15), Oct. 9, 1995, pp. 2111-2113; 2) U.S. Pat. No. 5,259,048, entitled "OPTICAL EQUALIZER"; and 3) U.S. Pat. No. 5,367,582, entitled "VERTTCALLY-COUPLED ARROW MODULATORS OR SWITCHES ON SILICON." U.S. Pat. Nos. 5,259,048 and 5,367,582 are hereby incorporated by reference into the specification of the present invention. These prior art articles and patents do not disclose the use of multiple gratings in a single fiber or non-fiber waveguide as does the present invention. Also, these prior art articles require the use of at least one additional fiber or non-fiber waveguide coupled to the original fiber or waveguide, while the present invention does not.
Another prior art method of adding delay or doing dispersion compensation is to transmit a signal over a fiber (or non-fiber waveguide), reflect the signal off of a first discrete grating element situated in free space for introducing delay or negative group velocity dispersion, and further reflecting the reflected signal off of a second discrete grating element situated in free space. The second reflection of the signal is delayed or compensated as desired. The drawbacks to this approach include 1) requiring at least three discrete elements, 2) having two of these elements located in free space which may be difficult to control, 3) inducing large signal power loss through grating diffraction inefficiency as well as coupling between the fiber or other waveguide and free-space, and 4) the available dispersion functions are limited to monotonically increasing or decreasing delay rather than user definable as in the present invention. This approach has been disclosed in the following book, articles, and patent: 1) "Fundamentals of optical fibers," by Buck, pp. 227-228, 1995, John Wiley & Sons, New York, N.Y.; 2) "New Diffraction Grating Pair With Very Linear Dispersion For Laser Pulse Compression," by Tournois, published in "Electronics Letters," Aug. 5, 1993, Vol. 29, No. 16, pp. 1414-1415; 3) "Grating Compensation of Third-Order Fiber Dispersion," by Stern et al., published in "IEEE Journal of Quantum Electronics," Vol. 28, No. 12, December 1992, pp. 2742-2748; 4) U.S. Pat. No. 5,363,226, entitled "APPARATUS AND METHOD FOR DISPERSION COMPENSATION FOR A GRATING COUPLER USING A SURFACE RELIEF REFLECTION GRATING." U.S. Pat. No. 5,363,226 is hereby incorporated by reference into the specification of the present invention. These prior art articles, book, and patent do not disclose the use of multiple gratings in a single fiber, or nonfiber waveguide, as does the present invention.
Prior art articles disclose the use of a single, chirped grating in a waveguide for dispersion compensation. These articles include: 1) "Dispersion cancellation using linearly chirped Bragg grating filters in optical waveguides," by Ouellette, published in "Optics Letters," Vol. 12, No. 10, October 1987, pp. 847-849; and 2) "Waveguide Grating Filters for Dispersion Compensation and Pulse Compression," by Roman et al., published in "IEEE Journal of Quantum Electronics," Vol. 29, No. 3, March 1993, pp. 975-982. These articles do not disclose the device of the present invention. Additionally, these articles disclose, ultimately, sending the compensated signal backward in the fiber toward the source, while the present invention does not.
The use of a single grating to transfer the signal from one fiber mode to another is disclosed in the prior art in the following articles: 1) "Mode-coupling characteristics of UV-written Bragg gratings in depressed-cladding fibre," by Morey et al., published in "Electronics Letters," Apr. 28, 1994, Vol. 30, No. 9, pp. 730-732; 2) "Long-Period Fiber Gratings as Band-Rejection Filters," by Vengsarkar et al., published in "Journal of Lightwave Technology, Vol. 14, No. 1, January 1996, pp.58-65; and 3) "Efficient Mode Conversion In Telecommunication Fibre Using Externally Written Gratings," by Hill et al., published in "Electronics Letters," Aug. 2, 1990, Vol. 26, No. 16, pp. 1270-1272. These prior art articles do not disclose the device of the present invention.
Other prior art articles and patents disclose the use of multiple gratings but with additional, and sometimes expensive, components (e.g., a circulator) not required by the present invention or use the multiple gratings for devices that do not add delay or compensate for dispersion using a single fiber, or non-fiber waveguide, as does the present invention. These articles and patents include: 1) "Chirped in-fiber Bragg gratings for compensation of optical-fiber dispersion," by Hill et al., published in "Optics letters," Vol. 19, No. 17, Sep. 1, 1994, pp. 1314-1316; 2) "Fiber Gratings As Demultiplexing Filters For WDMA Networks," by Pastor et al., published by The Institution of Electrical Engineers, 1995, pp. 13/1-13/6; 3) U.S. Pat. No. 5,404,413, entitled "OPTICAL CIRCULATOR FOR DISPERSION COMPENSATION"; 4) U.S. Pat. No. 5,434,702, entitled "OPTICAL REPEATERS"; and 5) U.S. Pat. No. 5,093,876, entitled "WDM SYSTEMS INCORPORATING ADIABATIC REFLECTION FILTERS." U.S. Pat. Nos. 5,404,413; 5,434,702; and 5,093,876 are hereby incorporated by reference into the specification of the present invention.
U.S. Pat. No. 5,613,028, entitled "CONTROL OF DISPERSION IN AN OPTICAL WAVEGUIDE," discloses a device for and method of compensating for dispersion by keeping the signal in a single propagating mode, constructing the waveguide having alternating sections of positive mode dispersion and negative mode dispersion. The present invention discloses a device for compensating for dispersion that does not keep the signal in a single propagating mode and does not employ a waveguide having alternating sections of positive mode dispersion and negative mode dispersion. U.S. Pat. No. 5,613,028 is hereby incorporated by reference into the specification of the present invention.